Introduction 



11 



strata, the use of Foraminifera is the most widely applied, with that of 

 ostracods second in application. Insoluble-residue work is adaptable 

 locally and has been of major assistance in correlating carbonate sections. 

 Detrital mineralogy has been minimized in stratal correlations; locally, 

 however, it has its value. Table 1 indicates the relative usage of the more 

 common methods of correlating subsurface strata in various areas. 



TABLE 1 



Relative Importance of the More Common Methods Used 

 IN Correlating Subsurface Strata in Various Areas 



Questions 



1. Define "subsurface geology." 



2. Why has it been necessary to improve subsurface geologic prac- 

 tices? 



3. Subsurface methods vary in nature and complexity. Why? 



4. Present-day petroleum-exploration programs require systematic 

 and accurate subsurface interpretation. These requirements have 

 improved what problems? 



5. What are some of the essential duties of the subsurface geologist? 



6. Review table 1 which concerns the relative importance of meth- 

 ods used for correlating strata in the subsurface. 



7. Compare the drilling records from 1927 to 1949. 



8. What is meant by "geologic masks"? Give several examples. 



